Laurel
The Lauraceae or Laurel family comprises a group of flowering plants included in the order Laurales. The family contains about 55 genera and over 2000 (perhaps as many as 4000) species world-wide, mostly from warm or tropical regions, especially Southeast Asia and Brazil. Most are aromatic evergreen trees or shrubs, but Sassafras and one or two other genera are deciduous, and Cassytha is a genus of parasitic vines. Trees of the laurel family predominate in the world's laurel forests, which occur in a few humid subtropical and mild temperate regions of the northern and southern hemispheres, including the Macaronesian islands, southern Japan, Madagascar, and central Chile. There are three main economical uses for this family. A high content of essential oils are found in many Lauraceae that are important for spices and perfumes. Avocados are important oil-rich fruit that are now planted in warm climates across the world. The hard wood of several species is a source for timber around the world. The following genera include species with commercial value and are consequently among the best known: * Cinnamomum: Cinnamon, Cassia and Camphor Laurel * Laurus: Bay Laurel * Lindera: Spicebush * Persea: Avocado * Sassafras: Sassafras Classification Classification within the Lauraceae remains unresolved. Multiple classification schemes base on a variety of morphological and anatomical characteristics have been proposed but none are fully accepted. According to Judd et al. (2007), the suprageneric classification proposed by van der Werff and Richter (1996) is currently the authority. However, due to an array of molecular and embryological evidence that disagrees with the groupings, it is not fully accepted by the scientific community. Their classification is based on both inflorescence structure and wood and bark anatomy. It divides Lauraceae into two subfamilies, Cassythoideae and Lauroideae. The Cassythoideae comprises a single genus, Cassytha, and is defined by its herbaceous, parasitic habit. The Lauroidaeae is then divided into three tribes: Laureae, Perseeae, and Cryptocaryeae. The subfamily, Cassythoideae, is not fully supported. Backing has come from matK sequences of chloroplast genes while a questionable placement of Cassytha has been concluded from analysis of intergenetic spacers of chloroplast and nuclear genomes. Embryological studies also appear contradictory. One study by Heo et al. (1998) supports the subfamily. It found that Cassytha develops an ab initio cellular type endosperm and rest of the family (with one exception) develops a nuclear type endosperm. Kimoto et al. (2006) suggests that Cassytha should be placed in the Cryptocaryeae tribe because it shares a glandular anther tapetum and an embryo sac protruding from the nucellus with other members of the Cryptocaryeae. The Laureae and Perseeae tribes are not well supported by any molecular or embryological studies. Sequences of the matK chloroplast gene as well as sequences of chloroplast and nuclear genomes reveal close relationships between the two tribes. Embryological evidence does not support a clear division between the two tribes either. Genera such as Caryodaphnopsis and Aspidostemon that share embryological characteristics with one tribe and wood and bark characteristics or inflorescence characteristics with another tribe blur the division of the these groups. All available evidence, except for inflorescence morphology and wood and bark anatomy, fails to support separate Laureae and Perseeae tribes. The Cryptocaryeae tribe is partially supported by molecular and embryological studies. Chloroplast and nuclear genomes supports a tribal grouping that contains all the genera circumscribed by van der Weff and Richter (1996) as well as three additional genera. Partial support for the tribe is also attained from the matK sequences of chloroplast genes as well as embryology. Challenges in Lauraceae classification The knowledge of all individuals comprising the Lauraceae is incomplete. As of 1991, approximately 25-30% of neotropical Lauraceae species had not been described. As of 2001, embryological studies had only been completed on individuals from 26 genera yielding a 38.9% level of knowledge, in terms of embryology, for this family. Additionally, the huge amount of variation within the family for any potential defining characteristic poses a major challenge for developing a reliable classification. It is impossible to describe even one genus or tribe by a single well-defined character. For this reason, all proposed classifications rely on a set of characteristics where the combination presents the most frequently observed traits for the group. Genera (*: Machilus is often included in Persea as a subgenus) References Category:Laurel